Having made known to us the mystery of his will - The word “mystery” (μυστήριον mustērion) means literally something into which one must be “initiated” before it is fully known (from μυέω mueō, to initiate, to instruct); and then anything which is concealed or hidden. We commonly use the word to denote that which is above our comprehension or unintelligible. But this is never the meaning of the word in the New Testament. It means there some doctrine or fact which has been concealed, or which has not before been fully revealed, or which has been set forth only by figures and symbols. When the doctrine is made known, it may be as clear and plain as any other. Such was the doctrine that God meant to call the Gentiles, which was long concealed, at least in part, and which was not fully made known until the Saviour came, and which had been until that time “a mystery - a concealed truth” - though when it was revealed, there was nothing incomprehensible in it. Thus, in Colossians 1:26, “The mystery which hath been hid from ages and from generations, but now is made manifest to his saints.” So it was in regard to the doctrine of election. It was a mystery until it was made known by the actual conversion of those whom God had chosen. So in regard to the incarnation of the Redeemer; the atonement; the whole plan of salvation. Over all these great points there was a veil thrown, and people did not understand them until God revealed them. When they were revealed, the mystery was removed, and men were able to see clearly the manifestation of the will of God.
Which he hath purposed in himself - Without foreign aid or counsel. His purposes originated in his own mind, and were concealed until he chose to make them known; see 2 Timothy 1:9.